The present invention relates to transformer high voltage bushings and more particularly to an improved arrangement of a bushing, a fuse link, and a lightning arrester.
References believed to be relevent to the present invention include U.S. Pat. No. 2,453,719 issued to McFarlin in 1948 and U.S. Pat. No. 3,249,815 issued to Henry in 1966. Each of these patents generally illustrates the usefulness of lightning arresters with power distribution transformers. The Henry patent further shows the use of an explosive disconnector with a lightning arrester.
As demonstrated by the above-referenced patents, it is generally well known that the high voltage line inputs to power distribution transformers must be protected in several ways. Lightning arresters, otherwise known as excessive potential discharge devices, are normally positioned between the high tension line and a ground connection at the input to such transformers. As shown in the Henry patent, such protectors may be wired directly across the high tension line to ground circuit or may employ external spark gaps in addition to the normal internal spark gaps. As noted in the Henry patent, it is not at all uncommon for the lightning protectors themselves to fail and draw excessive currents from the high tension line with possible disruption of service resulting. The explosive disconnectors provided in the Henry arrangements are intended to separate the lightning arrester from the line upon such an occurrence.
It is additionally desirable, in most installations, to provide a fuse of some type between the high tension line and the input connection to the transformer windings. Neither of the above-referenced patents specifically show such fuse link. Quite often such fuse link comprises merely a reduced diameter length of wire, or weak link, connected to the end of the high tension bushing within the transformer casing. It is difficult to detect the fact that such a fuse link has separated and it is even more difficult to replace such a link.
Since the connection to the high tension side of the transformer is normally made at the upper end of the high tension bushing, any external fuse link must normally be connected between the bushing and the high tension line. To provide appropriate mounting locations for such an external fuse link, a stand-off insulator is often provided as an additional element attached to the top of a lightning arrester carried on a transformer case. Such a stand-off insulator is illustrated in FIGS. 6, 8 and 9 of the above-referenced McFarlin patent. The high tension line is normally connected to the top of the stand-off insulator and an external fuse link is supported between the stand-off insulator and the normal transformer high tension bushing. As a result, the stand-off bushing, high tension line connection and one end of the fuse, are supported by the lightning arrester.
As noted above, it is not unusual for lightning arresters to fail. In addition, it is not unusual upon such failure for the lightning arrester to physically disintegrate or at least break into several pieces. When the high tension line is supported by the lightning arrester, such mechanical breakage of the lightning arrester can disrupt service in several ways. Primarily, the broken pieces of the arrester which are then hanging from the high tension line tend to pull the line downwards and against the grounded transformer casing thereby shorting out the high tension line. If the fuse is positioned between the stand-off insulator and high tension bushing, such a short circuit is not interrupted by the fuse link. Even if a short to ground does not occur, damage or breaking of the fuse link can occur which will interrupt service to the transformer.
Thus, it is seen that in many circumstances the line protecting lightning arrester actually results in the precise disruption of service which it is supposed to avoid.